Microporous sheet product and methods for making and using the same

ABSTRACT

Microporous sheet product and methods of making and using the same. In one embodiment, the microporous sheet product is made by a process that includes melt-extruding a sheet material using an extrusion mixture that includes a thermoplastic polymer, a superabsorbent polymer, and a compatibilizing agent. After extrusion, the compatibilizing agent may be removed from the sheet material. When the sheet product is imbibed with a polar or ion-containing liquid, the superabsorbent polymer swells, causing a reduction in the pore size of the sheet product. The exposure also causes some of the superabsorbent polymer to migrate to the exterior of the microporous sheet product. The microporous sheet product may be used, for example, as a battery separator, as a food packaging material, as a diffusion barrier in the ultrafiltration of colloidal matter, and in disposable garments.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/327,218, inventor William Winchin Yen, filed Jan. 18, 2017,which is a National Stage Application under 35 U.S.C. 371 ofPCT/US2015/046060, filed Aug. 20, 2015, which, in turn, claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional patent ApplicationNo. 62/040,257, filed Aug. 21, 2014, and U.S. Provisional patentApplication No. 62/112,904, filed Feb. 6, 2015, the disclosures of allof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to microporous sheet productsand relates more particularly to a novel microporous sheet product andto methods of making and using the same.

Microporous sheet products are well-known and commonly used articlesfound in items as diverse as, for example, storage batteries, foodpackaging materials, and ultrafiltration devices. For example, instorage batteries, microporous sheet products are commonly used asbattery separators. Typically, a storage battery includes at least onepair of electrodes of opposite polarity and, in many cases, includes aseries of electrode pairs of alternating polarity. The current flowbetween the electrodes of each pair is maintained by an electrolyte.Depending on the nature of the battery system, the electrolyte may beacidic, alkaline, or substantially neutral. For example, in alkalinestorage batteries, which include, but are not limited to, primary,secondary, nickel, zinc and silver cells, the electrolyte is generallyan aqueous solution of potassium hydroxide. By contrast, in lead acidbatteries, the electrolyte is typically a sulfuric acid solution, and,in lithium rechargeables, the electrolyte is typically a lithium saltsolution in an aprotic organic solvent or solvent blend.

A battery separator is typically provided in a storage battery betweenadjacent electrodes of opposite polarity to prevent direct contactbetween the oppositely charged electrode plates since such directcontact would result in a short circuit of the battery. In general, itis highly desirable for the separator to possess one or more of thefollowing qualities: (i) to be thin and lightweight to aid in providinga battery of high energy density and specific energy; (ii) to have astructure that inhibits dendrite formation between the electrode plates;(iii) to have the ability to enhance the uptake of the electrolyticcomposition over the electrode plates and, in so doing, to promote asubstantially uniform distribution of the electrolytic composition overthe electrode plates (an effect generally referred to as wicking); (iv)to provide the property of freely permitting electrolytic conduction;and (v) to have a dimensionally stable structure even during thermalexcursions (internal or external heating). It is further highlydesirable for the separator to be made in an economical andenvironmentally safe manner while being substantially free of defects,such as pinholes and the like.

One known type of separator comprises a nonwoven fibrous material, thenonwoven fibrous material typically having a high porosity, an averagepore size of at least 10 microns, and low resistivity. An example ofsuch a separator is disclosed in U.S. Pat. No. 4,279,979, inventorsBenson et al., which issued Jul. 21, 1981, and which is incorporatedherein by reference. In particular, in the aforementioned patent, thereis disclosed a nonwoven fibrous substrate for a battery separator. Theaforementioned substrate, which is said to be for an alkaline batteryseparator, is made of a lightweight, porous, heat bonded, syntheticorganic sheet material having a basis weight of less than about 35 gsmand a thickness of less than about 200 microns. The major fibrouscomponent is synthetic pulp comprising thermoplastic polyolefin fibershaving a prefused microfibrillar structure similar to wood pulp. Theminor fibrous component is a high tenacity polyamide fiber having afiber length greater than about 6 mm. The heat bonding by partial fusionof the microfibrillar polyolefin is sufficient to impart to the sheetmaterial a wet tensile strength of at least 400 On width whilepermitting retention of air permeability of about 100 liters per minuteand more. The substrate is said to be particularly well-suited for usein nickel-zinc batteries.

Another known type of separator is disclosed in U.S. Pat. No. 4,283,442,inventors Machi et al., which issued Aug. 11, 1981, and which isincorporated herein by reference. In particular, in the aforementionedpatent, there is disclosed a method of producing a dimensionally stablebattery separator. The method is characterized by grafting acrylic acidand/or methacrylic acid onto a polyethylene film, treating the resultingmembrane with an aqueous alkaline solution, and drying the treatedmembrane under application of tension.

Still another known type of separator comprises a microporous sheetproduct that is formed by extruding a composition that includes apolyolefin and a liquid plasticizer and, thereafter, removing theplasticizer to produce a sheet with a microporous structure. An exampleof such a separator is disclosed in U.S. Patent Application PublicationNo. US 2013/0029126 A1, inventor Yen, which was published Jan. 31, 2013,and which is incorporated herein by reference. In particular, in theaforementioned publication, there is disclosed a sheet product suitablefor use as a battery separator, as well as a method of forming the sheetproduct. The method comprises forming a mixture of a polyolefin and afluid having a high vapor pressure, shaping the mixture into a sheetmaterial and subjecting the sheet material to stretching/fluidvaporization at high temperature to form an intermediate material havinga ratio of percent fluid to percent polymer crystallinity of between0.15 and 1, followed by a second stretching/fluid vaporization at alower temperature while removing a portion of the remainder of the fluidfrom the sheet. The resultant sheet is annealed and the remainder offluid is removed to form a sheet product having a thickness comprising astratified structure of small and larger pore layered configurationacross its thickness.

Still yet another known type of separator is disclosed in U.S. Pat. No.8,722,231 B2, inventors Brilmyer et al., which issued May 13, 2014, andwhich is incorporated herein by reference. In the aforementioned patent,there is disclosed a separator for a lead-acid energy storage cell. Theseparator includes a microporous matrix of pore forming particles orfibers, the pore forming particles or fibers being made of natural andsynthetic rubbers, polyolefins (such as polyethylene), and non-wovenglass fibers. The separator further includes a reversibleporosity-controlling agent randomly distributed throughout themicroporous matrix. The reversible porosity-controlling agent may beselected from particles that expand or contract in response to anelectrolyte concentration or materials that expand or contract inresponse to temperature. The separator may also include a particulatefiller, which may be selected from carbon black, diatomaceous earth andsilica particles.

Additional documents that may be of interest include the following, allof which are incorporated herein by reference: U.S. Pat. No. 8,859,129B2, inventors Brilmyer et al., issued Oct. 14, 2014; U.S. Pat. No.8,728,659 B2, inventors Armacanqui et al., issued May 20, 2014; U.S.Pat. No. 8,690,981 B2, inventor Mao, issued Apr. 8, 2014; U.S. Pat. No.8,133,840 B2, inventors Mika et al., issued Mar. 13, 2012; U.S. Pat. No.8,129,450 B2, inventors Wood et al., issued Mar. 6, 2012; U.S. Pat. No.7,754,387 B2, inventors Harada et al., issued Jul. 13, 2010; U.S. Pat.No. 6,726,732 B2, inventors Kim et al., issued Apr. 27, 2004; U.S. Pat.No. 6,559,195 B1, inventors Yamamoto et al., issued May 6, 2003; U.S.Pat. No. 6,396,682 B1, inventors Kim et al., issued May 28, 2002; U.S.Pat. No. 5,922,417, inventors Singleton et al., issued Jul. 13, 1999;U.S. Pat. No. 5,478,677, inventors Choi et al., issued Dec. 26, 1995;U.S. Pat. No. 4,614,575, inventors Juda et al., issued Sep. 30, 1986;U.S. Pat. No. 4,330,602, inventors O'Rell et al., issued May 18, 1982;U.S. Pat. No. 4,288,503, inventor Goldberg, issued Sep. 8, 1981; U.S.Pat. No. 4,224,394 A, inventor Schmidt, issued Sep. 23, 1980; U.S. Pat.No. 4,100,324, inventors Anderson et al., issued Jul. 11, 1978; U.S.Patent Application Publication No. US 2015/0118540 A1, inventorsFujiwara et al., published Apr. 30, 2015; U.S. Patent ApplicationPublication No. US 2013/0034769 A1, inventors Takagi et al., publishedFeb. 7, 2013; U.S. Patent Application Publication No. US 2011/0081601A1, inventors Weber et al., published Apr. 7, 2011; U.S. PatentApplication Publication No. US 2010/0028758 A1, inventors Eaves et al.,published Feb. 4, 2010; U.S. Patent Application Publication No.2009/0142657 A1, inventor Yen, published Jun. 4, 2009; PCT InternationalPublication No. WO 94/20995 A2, published Sep. 15, 1994; Japanese PatentDocument No. JP 2000260413 A, published Sep. 22, 2000; Japanese PatentDocument No. JP 4746830 B2, published Aug. 10, 2011; Japanese PatentDocument No. JP 4746797 B2, published Aug. 10, 2011; Japanese PatentDocument No. 4371670 B2, published Nov. 25, 2009; United Kingdom PatentApplication Publication No. GB 838468 A, published Jun. 22, 1960; UnitedKingdom Patent Application Publication No. GB 790098A, published Feb. 5,1958; Ulbricht, “Advanced functional polymer membranes,” Polymer,47:2217-62 (2006); Rohatgi et al., “Separator Membrane from CrosslinkedPoly(Vinyl Alcohol) and Poly(Methyl Vinyl Ether-alt-Maleic Anhydride),”Nanomaterials, 5:398-414 (2015); Mendelsohn et al., “Fabrication ofMicroporous Thin Films from Polyelectrolyte Multilayers,” Langmuir,16:5017-23 (2000); and Wu et al., “Novel Microporous Films and TheirComposites,” Journal of Engineered Fibers and Fabrics, 2(1):49-59(2007).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel microporoussheet product.

According to one aspect of the invention, there is provided amicroporous sheet product, the microporous sheet product being made by amethod comprising melt-extruding an extrusion mixture to produce a sheetmaterial, the extrusion mixture comprising a thermoplastic polymer, asuperabsorbent polymer, and a compatibilizing agent, the compatibilizingagent promoting mixing of the thermoplastic polymer and thesuperabsorbent polymer and forming micropores in the sheet material.

In a more detailed feature of the invention, the thermoplastic polymermay comprise one or more thermoplastic polymers selected from the groupconsisting of polyolefins, polyamides, polyethylene terephthalate,polyacrylics, and polyvinyl acetate.

In a more detailed feature of the invention, the thermoplastic polymermay comprise one or more thermoplastic polymers selected from the groupconsisting of polyolefins and polyamides.

In a more detailed feature of the invention, the thermoplastic polymermay be a polyolefin.

In a more detailed feature of the invention, the thermoplastic polymermay be a polyethylene.

In a more detailed feature of the invention, the thermoplastic polymermay be a polyamide.

In a more detailed feature of the invention, the thermoplastic polymermay constitute about 15-80% by volume of the extrusion mixture.

In a more detailed feature of the invention, the superabsorbent polymermay comprise one or more superabsorbent polymers selected from the groupconsisting of cross-linked polyacrylates, methacrylates,polyacrylamides, carboxymethyl celluloses, polyvinyl alcohol copolymers,polyethylene oxides, starch-grafted copolyacrylates or polyacrylamides,and ethylene maleic anhydride copolymers.

In a more detailed feature of the invention, the superabsorbent polymermay comprise a cross-linked polyacrylate.

In a more detailed feature of the invention, the cross-linkedpolyacrylate may be a cross-linked lithium polyacrylate.

In a more detailed feature of the invention, the superabsorbent polymermay be in particle form and may have a particle size smaller than about30 microns.

In a more detailed feature of the invention, the superabsorbent polymermay have a particle size of between 1 to 10 microns.

In a more detailed feature of the invention, the superabsorbent polymermay constitute about 1-80% by volume of the extrusion mixture.

In a more detailed feature of the invention, the superabsorbent polymermay constitute about 22-40% by volume of the extrusion mixture.

In a more detailed feature of the invention, the compatibilizing agentmay be selected from the group consisting of plasticizers andsurfactants.

In a more detailed feature of the invention, the plasticizer may beselected from the group consisting of polyethylene oxide, polyethyleneglycol, hydroxypropylene, phthalates, mineral oil, and mineral spirits.

In a more detailed feature of the invention, the plasticizer may bemineral spirits.

In a more detailed feature of the invention, the compatibilizing agentmay constitute about 1-80% by volume of the extrusion mixture.

In a more detailed feature of the invention, the compatibilizing agentmay constitute about 5-70% by volume of the extrusion mixture.

In a more detailed feature of the invention, the compatibilizing agentmay constitute about 10-50% by volume of the extrusion mixture.

In a more detailed feature of the invention, the thermoplastic polymermay constitute about 15-80% by volume of the extrusion mixture, thesuperabsorbent polymer may constitute about 1-80% by volume of theextrusion mixture, and the compatibilizing agent may constitute about1-80% by volume of the extrusion mixture.

In a more detailed feature of the invention, the extrusion mixture mayfurther comprise an inorganic oxide.

In a more detailed feature of the invention, the inorganic oxide mayconstitute about 0-20% by volume of the extrusion mixture.

In a more detailed feature of the invention, the superabsorbent polymermay have a solubility parameter above 11, and at least one of thethermoplastic material and the compatibilizing agent may have asolubility parameter above 11.

In a more detailed feature of the invention, the thermoplastic polymermay have a solubility parameter above 11.

In a more detailed feature of the invention, the compatibilizing agentmay have a solubility parameter above 11.

In a more detailed feature of the invention, the method may furthercomprise removing the compatibilizing agent from the sheet material.

In a more detailed feature of the invention, the step of removing thecompatibilizing agent may comprise vaporizing the compatibilizing agent.

In a more detailed feature of the invention, the microporous sheetproduct may have a resistivity below 1,000 ohm-cm as tested in a 30% KOHsolution.

In a more detailed feature of the invention, the microporous sheetproduct may have a resistivity below about 500 ohm-cm as tested in a 30%KOH solution.

In a more detailed feature of the invention, the microporous sheetproduct may have a resistivity below 100 ohm-cm as tested in a 30% KOHsolution.

In a more detailed feature of the invention, the microporous sheetproduct may experience a weight loss of at least 20% of thesuperabsorbent polymer when soaked in 30% KOH solution for 2 days.

In a more detailed feature of the invention, the microporous sheetproduct may have an average pore size below 5 microns.

According to another aspect of the invention, there is provided amicroporous sheet product made by a method comprising melt-extruding anextrusion mixture, the extrusion mixture comprising a thermoplasticpolymer and a superabsorbent polymer, wherein the thermoplastic polymeris a polyamide.

In a more detailed feature of the invention, the superabsorbent polymermay be in particle form and may have a particle size smaller than about30 microns.

In a more detailed feature of the invention, the superabsorbent polymermay have a particle size of between 1 to 10 microns.

In a more detailed feature of the invention, the polyamide mayconstitute about 60-78% of the extrusion mixture, and the superabsorbentpolymer may constitute the remainder of the extrusion mixture.

In a more detailed feature of the invention, the extrusion mixture mayfurther comprise a compatibilizing agent to promote mixing of thethermoplastic polymer and the superabsorbent polymer.

In a more detailed feature of the invention, the compatibilizing agentmay constitute about 10-50% by volume of the extrusion mixture.

In a more detailed feature of the invention, the method may furthercomprise cooling the melt-extrudate and then subjecting the cooledmelt-extrudate to a stretching/liquid vaporization step.

In a more detailed feature of the invention, the microporous sheetproduct may have a resistivity below 1,000 ohm-cm as tested in a 30% KOHsolution.

In a more detailed feature of the invention, the microporous sheetproduct may have a resistivity below about 500 ohm-cm as tested in a 30%KOH solution.

In a more detailed feature of the invention, the microporous sheetproduct may have a resistivity below 100 ohm-cm as tested in a 30% KOHsolution.

In a more detailed feature of the invention, the microporous sheetproduct may experience a weight loss of at least 20% of thesuperabsorbent polymer when soaked in 30% KOH solution for 2 days.

According to yet another aspect of the invention, there is provided amultilayer sheet product, the multilayer sheet product comprising aplurality of stacked layers, wherein at least one of the stacked layersis any of the microporous sheet products described above.

According to still another aspect of the invention, there is provided amultilayer sheet product, the multilayer sheet product comprising afirst layer and a second layer, the first layer and the second layerbeing in direct contact with one another, the first layer comprising anyof the microporous sheet products described above, the second layerbeing devoid of a superabsorbent polymer.

It is another object of the present invention to provide a novel methodfor preparing a microporous sheet product.

According to one aspect of the invention, there is provided a method ofmaking a microporous sheet product, the method comprising the steps of(a) melt-extruding an extrusion mixture to produce a sheet material, theextrusion mixture comprising a thermoplastic polymer, a superabsorbentpolymer, and a compatibilizing agent, the compatibilizing agentpromoting mixing of the thermoplastic polymer and the superabsorbentpolymer and forming micropores in the sheet material, (b) then, coolingthe sheet material, and (c) then, subjecting the sheet material to astretching/vaporizing step, whereby the compatibilizing agent is removedfrom the sheet material, thereby producing a microporous sheet productcomprising an open-celled matrix of thermoplastic polymer in which thesuperabsorbent polymer is dispersed.

In a more detailed feature of the invention, the superabsorbent polymermay be a cross-linked lithium polyacrylate.

In a more detailed feature of the invention, the superabsorbent polymermay have a solubility parameter above 11, and at least one of thethermoplastic polymer and the compatibilizing agent may have asolubility parameter above 11.

It is another object of the present invention to provide a novel foodpackaging material and a method of preparing the same.

According to one aspect of the invention, there is provided a method ofpreparing a food packaging material, the method comprising the steps of(a) providing a microporous sheet product, the microporous sheet productmade by a method comprising melt-extruding an extrusion mixture toproduce a sheet material, the extrusion mixture comprising athermoplastic polymer, a superabsorbent polymer, and a compatibilizingagent, the compatibilizing agent promoting mixing of the thermoplasticpolymer and the superabsorbent polymer and forming micropores in thesheet material; and (b) then, imbibing the microporous sheet productwith a liquid smoke extract flavoring.

According to another aspect of the invention, there is provided a foodpackaging material made by the above-described method.

The present invention is also directed at a method of separating theelectrodes of a battery, the method comprising positioning, between theelectrodes, a microporous sheet product made by a method comprising (a)melt-extruding an extrusion mixture to produce a sheet material, theextrusion mixture comprising a thermoplastic polymer, a superabsorbentpolymer, and a compatibilizing agent, the compatibilizing agentpromoting mixing of the thermoplastic polymer and the superabsorbentpolymer and forming micropores in the sheet material and (b) then,removing the compatibilizing agent from the sheet material.

The present invention is further directed at a method of packaging afood item, the method comprising contacting the food item with amicroporous sheet product made by a method comprising melt-extruding anextrusion mixture to produce a sheet material, the extrusion mixturecomprising a thermoplastic polymer, a superabsorbent polymer, and acompatibilizing agent, the compatibilizing agent promoting mixing of thethermoplastic polymer and the superabsorbent polymer and formingmicropores in the sheet material.

Additional objects, as well as aspects, features and advantages, of thepresent invention will be set forth in part in the description whichfollows, and in part will be obvious from the description or may belearned by practice of the invention. In the description, reference ismade to the accompanying drawings which form a part thereof and in whichis shown by way of illustration various embodiments for practicing theinvention. The embodiments will be described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that structuralchanges may be made without departing from the scope of the invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is best definedby the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are hereby incorporated into andconstitute a part of this specification, illustrate various embodimentsof the invention and, together with the description, serve to explainthe principles of the invention. In the drawings wherein like referencenumerals represent like parts:

FIG. 1 is a scanning electron microscope (SEM) image, taken as a sideview, of a microporous sheet product suitable for use as, for example, abattery separator, the microporous sheet product being preparedaccording to the present invention;

FIG. 2 is a schematic side view of a microporous sheet product suitablefor use as, for example, a battery separator, the microporous sheetproduct being prepared according to the present invention and beingshown prior to being imbibed with a polar or ion-containing liquid;

FIG. 3 is a schematic side view of the microporous sheet product of FIG.2, the microporous sheet product being shown subsequent to being imbibedwith a polar or ion-containing liquid;

FIG. 4 is a schematic side view of a comparative sheet product to themicroporous sheet product of FIG. 2, the comparative sheet product beingmade using an extrusion mixture lacking a compatibilizing agent, thecomparative sheet product being shown prior to being imbibed with apolar or ion-containing liquid;

FIG. 5 is a schematic side view of the comparative sheet product of FIG.4, the comparative sheet product being shown subsequent to being imbibedwith a polar or ion-containing liquid; and

FIG. 6 is a schematic side view of a multi-layer microporous sheetproduct constructed according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed at a novel microporous sheet product,as well as to methods of making and using the same. The presentinvention is based, in part, on the surprising discovery that amicroporous sheet product having desirable properties can be prepared bymelt-extruding an extrusion mixture to produce a sheet material, theextrusion mixture comprising a thermoplastic polymer, a superabsorbentpolymer, and a compatibilizing agent. The compatibilizing agent promotesmixing between the thermoplastic polymer and the superabsorbent polymerand, in addition, creates (i.e., by phase-separation) micropores in thesheet material, the compatibilizing agent substantially filling themicropores of the sheet material to produce a “wet” porous structure. Incertain instances, for example, where the microporous sheet material isused as a food packaging material, the thus-produced microporous sheetmaterial may be used without any further processing. In other instances,for example, where the microporous sheet material is used as a batteryseparator, the microporous sheet material may thereafter be processed toextract the compatibilizing agent from the sheet material, therebyproducing a “dry” porous structure. In any event, whether the porousstructure is “wet” or “dry,” the resultant porous structure comprises anopen-celled matrix of thermoplastic polymer in which superabsorbentpolymer particles are dispersed. When the microporous sheet product ofthe present invention is thereafter exposed to a polar or ion-containingsolvent, the superabsorbent polymer particles absorb the solvent andswell. Such swelling causes a reduction in the pore size of themicroporous sheet product, as well as an increase in theelectrolytic-conductivity of the microporous sheet product.Surprisingly, the absorption of solvent by the superabsorbent polymerparticles also causes some of the superabsorbent polymer particles tomigrate irreversibly from within the matrix to the exterior of thematrix whereas other superabsorbent polymer particles remain within thematrix. Those superabsorbent particles that migrate from within thematrix to the exterior of the matrix tend to increase the hydrophilicityof the exterior surfaces of the sheet product.

For purposes of clarity, some of the terms used herein and in theappended claims to describe the subject invention are explained furtherbelow:

The term “sheet material” is intended to refer to a unitary articlehaving two large surfaces with respect to its length and breadthdimensions and having a thickness between said surfaces. In general, theterm is used to describe structures achieved during the initialextrusion or shaping of material into a sheet-like form and ofstructures produced during subsequent processing of the sheet material.

The term “sheet product” is intended to encompass a single-layer ormulti-layer structure consisting of a single sheet material orcomprising a plurality of stacked or laminated sheet materials.

The terms “fluid,” “liquid,” or “solvent,” used interchangeably, referto liquid components used in the extrusion mixture used to form sheetmaterial. These terms may also be used in reference to a liquid used ina cooling bath for initial cooling of a formed sheet material, fluidused in other processing steps, and for the fluid removed during astretching/fluid vaporization step.

The term “separator” is intended to refer to a component of a battery,in particular a storage battery, by which the component maintains aseparation between adjacent electrode plates or elements of oppositepolarity. The separator may be of various configurations, such as flat(preferred), ribbed, corrugated sheet which may be in the form of amembrane or envelope capable of maintaining separation of adjacentelectrodes.

The term “dendrite” is intended to refer to growths that develop on andextend outward from the surface of an electrode element and are due tothe re-plating of electrode material during cycling of the battery.Dendrite formations that traverse through a separator from one electrodeto another electrode of opposite polarity may cause shorting of thebattery cell.

The term “fluidity” is intended to refer to polymeric compositions thatexhibit flow properties that are caused by the physical ability of thepolymer molecules of the composition to slide over one another. Thisability is enhanced by the inclusion of a fluid material, especiallywhen the polymer has minor (low) solubility properties with respect tothe fluid component in contact therewith.

The terms “superabsorbent polymer,” “SAP,” and “superabsorber” refer toa polymeric compound which can absorb and retain large amounts of liquidrelative to its own mass. The superabsorbent polymer creates intersticesfrom the absorbing liquid.

The term “solubility parameter” refers to a numerical estimate of thedegree of interaction between materials and is a good indication ofmaterial compatibility, particularly for nonpolar materials, such asmany polymers. The solubility parameter for polyolefin is typicallyabout 8, for paraffin oil is about 8, for nitrile rubber is about 9, forpolyester is about 11, for polyamide is about 14, for water is 23.4, andfor ethylene glycol is about 30.

$\delta = \sqrt{\frac{{\Delta H}_{v} - {RT}}{V_{m}}}$

δ=solubility parameter, [call/2 cm-3/2]

H_(v)=heat of vaporization

R=gas constant

T=temperature

V_(m)=molar volume of molecules in the condensed phase

As noted above, the microporous sheet product of the present inventionmay be formed, at least in part, by melt-extruding an extrusion mixture,the extrusion mixture comprising a thermoplastic polymer, asuperabsorbent polymer, and a compatibilizing agent, the compatibilizingagent promoting mixing between the thermoplastic polymer and thesuperabsorbent polymer and also creating micropores in the resultantsheet material.

The thermoplastic polymer of the above-described extrusion mixture maybe used primarily as a binder to provide a supporting scaffold or matrixin which superabsorbent polymer particles may be dispersed and toprotect the superabsorbent polymer from thermal degradation duringmelt-extrusion. As such, the thermoplastic polymer may comprise one ormore thermoplastic polymers of the type that can be used to form amicroporous sheet by melt-extrusion. The one or more thermoplasticpolymers may include one or more thermoplastic homopolymers, copolymersor terpolymers. The thermoplastic polymer of the present inventionpreferably has a weight average molecular weight of from about 20,000 toabout 1,000,000. Examples of suitable classes of thermoplastic polymersmay include, but are not limited to, polyolefins, polyamides,polyethylene terephthalate, polyacrylics, polyvinyl acetate, and thelike. Preferred classes of thermoplastic polymers include polyamides andpolyolefins. Examples of polyolefins include, but are not limited to,linear low density or high density polyethylene, polypropylene, andpolybutylene.

Melt flow index or MFI is a measure of the ease of flow of the melt of athermoplastic polymer. It is defined as the mass of polymer, in grams,flowing in ten minutes through a capillary of a specific diameter andlength by a pressure applied via prescribed alternative gravimetricweights for alternative prescribed temperatures. The method fordetermining MFI is described in ASTM D1238 and ISO 1133. Melt flow rateis an indirect measure of the molecular weight of a polymer. Preferredpolyolefins for use as the thermoplastic polymer of the presentinvention have a Melt Flow Index (MFI) below about 4.

Preferred polyamides for use as the thermoplastic polymer of the presentinvention typically have formic acid Relative Viscosity (RV) of 4 andabove in accordance with ISO 307.

Preferred polyolefins for use as the thermoplastic polymer of thepresent invention have a crystallinity of at least 30 percent.

As noted above, the thermoplastic polymer component of the presentinvention may comprise multiple polymers. For example, a polyethylenemay be blended with a second polymer, such as a polypropylene, apolyamide, a fluoropolymer, or the like, in varying percentages.Alternatively, a blend of different molecular weight grades of the samepolymer (e.g., a polyethylene) may be used.

The thermoplastic polymer preferably constitutes about 15-80% by volumeof the extrusion mixture.

The superabsorbent polymer of the above-described extrusion mixture maybe used primarily to absorb liquid that may be added to the microporoussheet product. For example, in those instances in which the microporoussheet product is used as a battery separator, the superabsorbent polymermay be used to absorb liquid electrolyte. In so doing, thesuperabsorbent polymer may reduce the pore size of the thermoplasticpolymer matrix, as well as improving the wettability of the batteryseparator, allowing electrolytic conduction, and reducing the resistanceof the battery separator.

The superabsorbent polymer of the present invention may comprise one ormore types of superabsorbent polymers. Examples of suitablesuperabsorbent polymers include, but are not limited to, variouscross-linked polymers, such as cross-linked polyacrylates (e.g., sodium,hydrogen, potassium, lithium, or zinc), methacrylates, polyacrylamides,carboxymethyl celluloses, polyvinyl alcohol copolymers, polyethyleneoxides, starch-grafted copolyacrylates or polyacrylamides, ethylenemaleic anhydride copolymers, and copolymers thereof. The superabsorbentpolymer may further include a functional cation, such as a lithium ion,a sodium ion, a potassium ion, an alkaline earth metal ion, or a zincion. The superabsorbent polymer is preferably in particle form andpreferably has a particle size smaller than about 100 microns, morepreferably smaller than about 30 microns, and most preferably between 1to 10 microns. In this manner, by keeping the particle size of thesuperabsorbent polymer small, it is easier to evenly distribute thesuperabsorbent polymer throughout the extruded sheet material, therebyreducing the frequency of pinholes being formed in the resultant sheetproduct. Organic acid superabsorbers, such as, but not limited to,polyacrylic acid, polymethacrylic acid, and ethylene maleic anhydridepolymer, tend to best absorb liquid near neutral pH. The extent ofcross-linking in the superabsorbent polymer should be kept withinspecific limits so that the superabsorbent may absorb liquids, such as aliquid electrolyte, without forming an amorphous gel.

The superabsorbent polymer preferably constitutes about 1-80% by volumeof the extrusion mixture.

The compatibilizing agent of the aforementioned extrusion mixture may beused to promote a uniform mixing of the thermoplastic polymer and thesuperabsorbent polymer and to help the mixture to flow at elevatedtemperatures (e.g., about 100-250° C.). In addition, the compatibilizingagent may also be used to create (i.e., by phase-separation) microporesin the extruded sheet material. Where the superabsorbent polymer has asolubility parameter above 11, it may be desirable for at least one ofthe thermoplastic polymer and the compatibilizing agent to have asolubility parameter above 11.

The compatibilizing agent of the present invention may be, for example,any plasticizer or surfactant that promotes the uniform mixing of thethermoplastic polymer and the superabsorbent polymer at elevatedtemperatures. In most cases, the compatibilizing agent is a liquid.Examples of plasticizers that may be used as a compatibilizing agentaccording to the present invention include, but are not limited to, lowmolecular weight organic liquids, such as mineral spirits, mineral oil,lower molecular weight alkanes, C₉-C₂₀ aliphatic, alicyclic or aromatichydrocarbons, polyethylene oxide, glycols (e.g., polyethylene glycol),hydroxypropylene, phthalates, oils, food additives, and the like, aswell as mixtures thereof.

The compatibilizing agent phase-separates from the thermoplastic andsuperabsorbent polymers and, in so doing, creates micropores in thethus-formed sheet material. In those instances where, for example, themicroporous sheet material is to be used as a food packaging material,no further processing of the microporous sheet material may be needed.On the other hand, in those instances where, for example, themicroporous sheet material is to be used, for example, as a batteryseparator, the microporous sheet material may thereafter be treated sothat the compatibilizing agent is removed from the microporous sheetmaterial. Such removal of the compatibilizing agent may be effected, forexample, by a conventional solvent extraction technique and/or by thestretching/vaporization technique of U.S. Patent Application PublicationNo. US 2013/0029126 A1. The removal of the compatibilizing agent in theaforementioned fashion creates open (available) capillaries forelectrolyte conductivity in the finished sheet product.

The compatibilizing agent preferably constitutes about 1-80% by volume,more preferably 5-70% by volume, and most preferably 10-50% by volume,of the mixture.

The above-described extrusion mixture may further contain small amountsof fillers, colorants, anti-oxidants, stabilizers, and the like. Forexample, the mixture may contain one or more inorganic oxides, which mayimprove the porosity and rate of swelling of the sheet product. (Wherethe sheet product is used as a battery separator, such swelling createsbeneficial cell stack pressure.) Suitable inorganic oxides may include,but are not limited to, oxides of silicon, aluminum, lithium, magnesium,calcium, titanium, zinc, zirconium, or barium. Such oxides may be in theform of fine particles, preferably about 0.1-10 microns in diameter.Such particles may have a surface area of at least 5 m²/g, andpreferably from about 5-200 m²/g and may have a pore volume (BET) offrom about 0.01-1 ml/g. The particles may be prepared by any method thatresults in fine particles, such as, but not limited to, milling,condensation, precipitation, fume condensation, or any other appropriatemethod. These compounds, when added to the extrusion mixture, increaseporosity, especially after stretching of the polymer sheet material.Preferably, the oxide is crystalline, is essentially insoluble inaqueous solution, especially basic solution, and has an elevated meltingtemperature of over 500° C.

Where an inorganic filler is used as part of the extrusion mixture, theinorganic filler preferably constitutes about 0-20% by volume of themixture.

As noted above, an extrusion mixture of the type described above may bemelt-extruded to produce a microporous sheet material, and, depending onthe use to which the microporous sheet material is intended to be put,the microporous sheet material may then undergo additional processing.Details of a suitable melt-extrusion process and of a suitable methodfor processing the extruded sheet material to produce a microporoussheet product suitable for use as a battery separator are providedbelow.

First, the extrusion mixture may be prepared and extruded. Mixing may beperformed prior to or during the extrusion process. For example, thevarious components of the extrusion mixture may be fed into a single- ortwin-screw feed chamber of an extruder. An example of a suitableextruder is disclosed in PCT International Publication No. WO2009/051278 A2, which was published on Apr. 23, 2009, and which isincorporated herein by reference. Preferably, a co-rotating twin screwcontinuous extruder is used to blend the various components of theextrusion mixture and to produce an extrudate. The extruder, whichpreferably has two shafts, preferably has at least a L/D (length overdiameter) of at least 24 and at least 5 barrels (temperature zones). Themixture may be fed into the first barrel, with the second barrel beingused to blend and to melt the mixture. Optionally, liquid plasticizermay be injected into a subsequent barrel, and an open barrel may be usedto evacuate any potential volatiles, such as water vapor, in themixture. Finally, a pumping barrel may be used to pressurize the meltand to pump the melted mixture from the extruder into a die on acontinuous basis. The extruder preferably comprises at least 3 L/D ofright-handed conveying screw bushing, at least 1/3 L/D of left-handedscrew bushing for conveying control, and at least 1 L/D of right-handedkneading blocks for the melting and mixing of materials. The meltedmaterial may then be pumped through a heated metal slotted die, whichmay be used to form the extrudate into a shaped film of sheet material.

The thus-formed sheet material may then be cooled. This may beaccomplished, for example, by casting the sheet material onto a chilledroll or by immersing the sheet material in a cooling bath for asufficient time to solidify the sheet material. The cooling roll or bathis preferably maintained at a temperature below 100° C. so that thesheet material is cooled below the melt temperature of the thermoplasticpolymer.

In certain instances, for example, where the sheet material is to beused as a food packaging material, it may be acceptable for thecompatibilizing agent to be retained in situ. In other instances, forexample, where the sheet material is to be used as a battery separator,the cooled sheet material may be subjected to some form of processing toremove the compatibilizing agent from the sheet material. Suchprocessing may involve a stretching/fluid vaporization technique thatmay comprise, in a first step, stretching the sheet in at least onedirection. This first direction of stretching may be conducted in themachine direction from which the sheet material exits the extrusion diehead and the cooling bath. The stretching can be readily accomplished,for example, by passing the sheet material through nip rollers of a setrotation surface speed and then through a second set of higher speed niprollers prior to a take-up roller. Alternately, other conventionalstretching means can be used, such as a tenter method, an inflationmethod or a combination thereof. The stretching in the first directionmay involve stretching the sheet material at least about 125% of itsinitial dimension in a first direction. This first stretching ispreferably done while maintaining the sheet material at an elevatedtemperature. For example, the stretching of polyethylene may be done attemperatures of about 20-150° C. depending on the polymer. Thestretching in the first direction may be accomplished in a one-stepoperation or in a series of stretching operations to achieve the desireddegree of elongation of the sheet material. Subsequent to stretching ina first direction, the sheet material may be stretched in a seconddirection, which may be transverse to the first stretching direction.The stretching in the second direction may comprise stretching fromabout 125-700% of its initial dimension in the second dimension. Thecombined stretches may have an overall ratio of about 1.5-70 fold.Depending on the formulation, machinery set-up, the stretching may beperformed in sequential monoaxial steps or simultaneous biaxialstretches.

Simultaneous to the stretching, a portion or all of the compatibilizingagent may be removed from the sheet material by vaporization. The easewith which a given compatibilizing agent may be vaporized will depend onthe volatility of the compatibilizing agent. Certain compatibilizingagents, such as mineral spirits, are highly volatile and can easily bevaporized by stretching at an elevated temperature.

With the stretched sheet material under tension in at least one or bothstretched directions, it may be subjected to annealing. The completenessof annealing is a function of heat-transfer effectiveness, temperature,residence time and relaxation. The stretched sheet material may bemaintained under these conditions for a period of at least 1 second. Thesheet material may be optionally relaxed in one or both stretcheddirections (length and/or width of about 5-20% reduction) as known inthe art to further improve dimensional stability.

In an alternative embodiment, the shaped sheet material may also beextruded in an annular die, forming the sheet in a continuous tubularform. The stretching orientation may be conducted in a conventionalsingle, double or triple bubble blown film equipment. The tubular filmmay be longitudinally stretched and simultaneously inflated to orientthe film under specific temperature.

A scanning electron microscope (SEM) image, taken from the side, of amicroporous sheet product prepared according to the present invention isshown in FIG. 1, the microporous sheet product being representedgenerally by reference numeral 11. As can be seen, microporous sheetproduct 11 comprises a matrix of thermoplastic polymer 13 and aplurality of superabsorbent polymer particles 15, the superabsorbentpolymer particles 15 being randomly distributed throughout the matrix ofthermoplastic polymer 13. The micropores 17 provided in microporoussheet product 11 represent the spaces that were previously occupied bythe compatibilizing agent, which has since been removed.

The thickness of a “dry” microporous sheet product (i.e., a microporoussheet product where the compatibilizing agent has been removed)according to the present invention, prior to being imbibed with anyliquid, may be about 0.1-20 mil (about 0.0025-0.50 mm) although thethickness may vary based on the particular application for which thesheet product is intended.

A single-layer “dry” sheet product according to the present invention,prior to being imbibed with any liquid, preferably has no macropores(above 10 microns), preferably has at least 10% of its volume incapillary porosity as micropores (under 1 micron), preferably has atleast 1% of superabsorber to create nanopores or interstices (under 0.1micron or even below 0.01 micron), is able to swell in thickness(thereby creating hydrostatic pressure after absorbing electrolyte) andis capable of absorbing at least 5%, preferably 10%, of its weight ofelectrolyte or another suitable liquid. The sheet product preferably hasa mean pore size below 5 microns, more preferably below 1 micron, with anarrow pore size distribution. By contrast, the superabsorbent polymerof the sheet product preferably has a particle size greater than 1micron. Consequently, the superabsorbent polymer preferably has aparticle size larger than the sheet product pore size. When thesuperabsorbent polymer is exposed to electrolyte or another suitablesolvent, it swells and expands within the smaller pores, therebyreducing the pore size of the sheet product, improving electrolytewettability of the pores and reducing the resistance of the sheetproduct.

Alternatively stated, a single-layer “dry” sheet product of the presentinvention, prior to being imbibed with any liquid, is a highly poroushomogeneous unitary or monolithic article. The sheet product has amicroporous structure preferably with an average pore size below 5microns, more preferably below 1 micron. The thickness or weight of thedry sheet product may increase by at least 5%, more preferably 10%,after exposure to a liquid for absorption (at standard temperature andpressure). The initial dry membrane may have a certain average pore sizeand porosity. This porosity increases after exposing the membrane to aliquid, filling the pores with liquid and creating additionalinterstices from absorption. On the other hand, the overall average poresize of the membrane may be reduced after liquid exposure. The number ofand average pore size may shift from micropores towards nanopores due tothe chemisorption of the superabsorbent. The pore size of the drymembrane as measured by the mercury intrusion method may be in thesubmicron range (about 1 micron), the pore size of the imbibed membraneas measured using methods like bubble point porosimetry may be reducedand get closer to the nanopore range (below 0.1 micron or smaller). Theaverage pore size may be reduced by 10% and more likely by 20% afterabsorbing liquid. Due to the chemisorption, liquid may preferentiallymigrate into the superabsorber, thus creating nanopores whilemaintaining dry or empty open micropores within the membrane structure,i.e., membrane is partially wetted in smaller pores for liquid iondiffusion and having larger pores partially dry for gas diffusion.

The porosity and pore size of the membrane surfaces allow capillaryabsorption of aqueous solutions, permitting chemisorption of electrolyteand liquid by the superabsorber.

Since capillary action is inversely proportional to pore size, theoptimal membrane configuration will have micropores allowingchemisorption of electrolyte.

The force of capillary action can be proportional to the capillarywicking height; the height h of a liquid column is given by:

${h = \frac{2\gamma\;\cos\;\theta}{\rho\;{\mathcal{g}}\; r}},$

where γ is the liquid-air surface tension (force/unit distance), θ isthe contact angle, ρ is the density of liquid (mass/volume), g is localacceleration due to gravity (distance/square of time), and r is radiusof tube (distance). Thus, the thinner the space (or pore size) in whichthe water can travel, the greater the column height.

On exposure to electrolyte, the superabsorber swells and formsnanoporous domains within its molecules, thereby allowing ionicconduction through the microporous sheet product. The liquid-absorbedsheet product may have a greater number and volume of nanopores than thenumber and volume of micropores before absorption, i.e., smaller averagesize filled interstices after absorption of liquid. The micro-porosityand chemisorption-based nanoporosity reduce electrochemical dendritepenetration or pinhole shorting by physical interference, yet allownormal and stable operating electrolyte conductivity.

The properties of sheet products useful as battery separators includenot only permeability, mechanical strength, and dimensional stability,but also properties related to electrolytic solution wicking,absorption, and battery cyclability. The present invention provides athin, lightweight sheet product that has high electrolyte retentioncapability, thereby providing the battery with the ability ofmaintaining electrolyte over the electrode surfaces and achieving highelectrolytic conductivity while, when appropriate, providing highinhibition to formation and growth of dendrites between electrodeelements of opposite polarity. At the same time, the sheet product ofthe present invention has high mechanical strength.

The sheet product structure disclosed here tends to swell in length,width and thickness, when exposed to a suitable liquid. It is desirableto have thickness swell and limited dimensional swell to create batteryelectrode stack pressure. The swelling of thickness is of importance inbattery systems. The advantages of this characteristic are multifold.During cycling, electrodes for rechargeable batteries are known toundergo expansion and contraction according to the incorporation of ionsinto the cathode structure and the stripping and replating of zinc (orlead or lithium) at the anode surface. Because separators are compressedwhen the electrodes expand, it is desirable that the separators have theability to undergo such compression while exhibiting, when compressed,as little a decrease as possible in electrolytic solution retention. Byhaving a separator which swells in electrolyte and maintains ahydrostatic pressure on the faces of the electrodes, it also maintainselectrochemical activity over the entire face of the electrode by notpermitting the formation of open pockets between the electrode surfaces.Since the superabsorbers used herein are somewhat gel-like, underpressure they deform to cover the whole electrode surface, maintainingintimate electrolyte and electrode contact, reducing overall cellresistance and, thus, improving electrochemical activity over thatsurface. SAP may swell and fill the capillary pores within theseparator, and some may migrate and expand onto the separator surface(s)to promote hydrophilic surface wetting, thereby improving electrolyteabsorption of the separator. An additional advantage accruing from thischaracteristic is that by maintaining electrochemical activity, it alsoguarantees a minimal current density (current per unit area of theelectrode face) and, consequently, optimal efficiency of electrodeactivity (the total capacity retrieved per unit electrode surface areaor mAh/cm²). The measure of resistivity (i.e. the inverse ofconductivity) is preferred to be below 1,000 ohm-cm, more preferablybelow about 500 ohm-cm, and most preferably below 100 ohm-cm, as testedin a 30% KOH electrolyte.

The degree of swelling of the separator exposed to electrolyte isoptimized so that the separator structure will absorb electrolyte bycapillary action and chemical attraction while not over-swelling orover-drying the cathode, rendering it inactive. The exact compositionand structure of the membranes described in this disclosure areengineered so that electrolyte absorption and retention may beoptimized.

Referring now to FIG. 2, there is shown a schematic side view of amicroporous sheet product prepared according to the present invention,the microporous sheet product being represented generally by referencenumeral 51.

Microporous sheet product 51, which is a “dry” membrane shown prior toits exposure to a polar or ion-containing liquid, comprises a matrix ofthermoplastic polymer 53 and a plurality of superabsorbent polymerparticles 55, the superabsorbent polymer particles 55 being randomlydistributed throughout the matrix of thermoplastic polymer 53. As can beseen, microporous sheet product 51 is a relatively open-celled structurehaving a plurality of micropores 57 extending from the top surface 59and bottom surface 61 of microporous sheet product 51. The open-celledstructure of microporous sheet product 51 may be attributable, at leastin part to the presence of a compatibilizing agent, such as aplasticizer or surfactant, in the extrusion mixture used to preparemicroporous sheet product 51. More specifically, after an extrudate isformed from an extrusion mixture containing such a compatibilizingagent, the compatibilizing agent may be vaporized or otherwise removed,with the spaces previously occupied by the compatibilizing agent leavingvoids. As can be appreciated, depending on the use to which microporoussheet product 51 is to be put, compatibilizing agent may not be removedand may be retained in the voids, thereby forming a “wet” membrane.

When microporous sheet product 51 is exposed to a polar orion-containing liquid, such as the liquid electrolyte of a storagebattery, not only do the superabsorbent polymer particles 55 absorb theliquid and swell, thereby reducing the size of many of the pores, butsome of the swollen superabsorbent polymer particles 55 irreversiblymigrate from within the matrix of thermoplastic polymer 53 to theexterior surfaces of the matrix, as is illustrated in FIG. 3. Thisirreversible migration of superabsorbent polymer particles 55 to theexterior surfaces of the sheet product increases the hydrophilicity ofthe sheet product.

By contrast, FIGS. 4 and 5 show a microporous sheet product 71 that wasmade from an extrusion mixture lacking a compatibilizing agent. As canbe seen, although microporous sheet product 71 also comprises a matrixof thermoplastic polymer 73 and a plurality of superabsorbent polymerparticles 75 dispersed in the matrix of thermoplastic polymer 73,microporous sheet product 71 has a more closed-cell structure than doesmicroporous sheet product 51. Moreover, as seen best in FIG. 5, whenmicroporous sheet product 71 is exposed to a polar or ion-containingliquid, such as the liquid electrolyte of a storage battery,substantially none of the superabsorbent polymer particles 75 migratefrom within the matrix of thermoplastic polymer 73 to the exteriorsurfaces thereof.

Without wishing to be limited to any particular theory behind theinvention, it is believed that the following discussion may shed somelight on how the invention may function when exposed to a polar orion-containing liquid, particularly as it applies to the use of theinvention as a battery separator. Electrolyte and water retention ofmembranes are controlled by the wettability of the membrane and the porestructure of the membrane. Smaller pore sizes of the membrane createhigher capillary action, thus higher liquid retention. The microporoussheet product of the present invention not only comprises small pores,it also has the ability to absorb and to store liquid in the createdinterstices within the superabsorbent, such capillary absorptionretaining electrolyte and liquids. The combination of molecular poresand the chemical structure of the superabsorber, providing the activegroups within the superabsorber molecules, aid in retention ofelectrolyte over the electrode faces and maintain electrolytic activityover the whole electrode surface. Absorption of electrolyte requiressome degree of cross-linking within the polymer structure, this limitssolvation of the superabsorbent polymer within the porous membranestructure; however, the swelling of the superabsorber createshydrostatic pressure and may fill the capillaries and pores with swelledsuperabsorber. The superabsorber may also migrate or expand onto themembrane surface to improve liquid absorption and wetting capability.Furthermore, the swelling of the superabsorber polymer may fill largerpinholes after absorbing electrolyte, thus preventing direct shortcircuiting between the electrodes within a cell. The superabsorber mayabsorb up to 500 times of its own weight or 500 times its volume.

The microporous sheet product of the present invention may consist of asingle layer of the type described above or may comprise a plurality ofstacked or laminated layers, one or more of which may be of the typedescribed above. A laminate structure may be readily formed usingconventional multi-sheet extrusion head devices (e.g. co-extrusion).Examples of multilayer structures are described in European PatentApplication Publication No. EP 1 911 352 A1, published Apr. 16, 2008,which is incorporated herein by reference. One or more of the layers ofa multilayer structure may be a protective layer, which may benon-porous to limit the permeability of pathogens or other detrimentalmicroorganisms and to improve film durability. Alternatively, themultilayer structure may comprise a middle layer comprising asuperabsorbent polymer and microporous outer layers not including thesuperabsorbent polymer. As the SAP swells upon exposure to electrolyte,it may migrate from the middle layer into an adjacent outer layer,promoting overall electrolyte wetting and conductivity. In addition tonot containing a superabsorbent polymer, the composition of the outerlayers may differ substantially from that of the inner layer, or,alternatively, the absence of a superabsorbent polymer may be the onlycompositional difference. In any event, the materials used in thevarious layers should be sufficiently compatible and miscible to permitadhesion during extrusion and juxtaposition of the layers.

Referring now to FIG. 6, there is shown a schematic side view of amultilayer microporous sheet product constructed according to thepresent invention, the multilayer microporous sheet product beingrepresented generally by reference numeral 101.

Multilayer microporous sheet product 101 comprises an inner layer 103and a pair of outer layers 105 and 107, with inner layer 103 beingsandwiched between outer layers 105 and 107. Inner layer 105 may beidentical in composition to microporous sheet product 51. Outer layers105 and 107 may be identical to one another and may differ from innerlayer 107 only in that outer layers 105 and 107 do not include asuperabsorbent polymer. Outer layers 105 and 107 may be microporous.

Layers 103, 105 and 107 may be laminated together, for example, byco-extrusion.

As can be appreciated, although multilayer microporous sheet product 101is of the BAB variety (layer A including the superabsorbent polymer andlayer B not including the superabsorbent polymer), multilayermicroporous sheet product 101 may be of the ABA variety, the AB variety,the ABC variety (with layer C differing in composition from both layersA and B), the ABCD variety (with layer D differing in composition fromlayers A, B and C), or other permutations.

In addition to use as a battery separator, the above-describedmicroporous membrane may be put to other uses. For some such uses, themembrane may be used without any further modification thereto whereas,for other uses, the membrane is preferably modified in some manner. Oneway in which the membrane may be modified is by being imbibed with amaterial that endows the membrane with a specific function. The imbibingmaterial may be a liquid or a dispersion of solid. Certain applicationsmay require two or more reactive components as imbibing materials topermit the reaction of the reactive components within the microporoussheet structure. Examples of imbibing materials include medicaments,fragrances, flavorings, colorants, antistatic agents, surfactants,antimicrobials, pesticides and solid particulate material, such asactivated carbon and pigments.

The microporous sheet product of the present invention may be laminatedto any of a variety of other structures, such as nonwoven, porous, andnon-porous sheet materials, to provide a composite structure. Nonwovenmaterials may include, but are not limited to, glass, cellulose,polyolefins, polyamide, polyester and other polymers. Lamination may beaccomplished by conventional techniques, such as coating, impregnation,adhesive bonding, spot-welding, or by other techniques which do notdestroy or otherwise interfere with porosity or which do not createundesirable porosity or perforations.

The microporous sheet product of the present invention may be employedin any of a wide variety of situations where microporous structures maybe utilized. The microporous sheet product may be used in theultrafiltration of colloidal matter, for example, as diffusion bathers.The membrane may be used as a geo-membrane, as a non-woven protectivescrim, disposable garment, or diaper, which may take advantage ofmoisture absorption from perspiration, and as disposable gloves.

Another application of the microporous membrane may be in the field offood packaging, such as in uncooked meat packaging, cooked meat andsausage casing, cheese packaging and specific applications thereof, toprovide flavor transfer and to promote adhesion. Other applications maybe fresh meat packaging, such as chicken shrink bags and ground beef andpork display tray liners, to absorb excess processing fluids. Oftenpackaged foods tend to expel moisture or blood from their solidstructure over time. Since the membranes described herein containsuperabsorbers within a microporous structure permitting moisture to beabsorbed instantaneously, the occurrence of pooled liquids within thefood packaging may be minimized. Other applications may includepackaging for fresh produce and bread, where equilibria of moisture,oxygen, and carbon dioxide levels should be attained to keep these foodsfresh for a longer shelve life.

Depending on the type of superabsorbent polymer, the SAP may absorborganic or aqueous liquids with a pH of 1 to 14. Some examples arelisted in Example 4 below. Liquid smoke flavor and color additives areespecially useful in cook-in food casing applications, where flavors canbe absorbed and desorbed onto meat products. A typical liquid smokeextract is Poly100 from Hickory Specialties. Liquid smoke extract in theacidic state may be neutralized to a basic pH to improve the absorptionof superabsorbent.

Still another possible application of the microporous sheet product ofthe present invention is with lithium ion electrochemical cells. Morespecifically, by neutralizing a polyacrylate with lithium hydroxide andthen cross-linking the polymer, a special version of lithiumpolyacrylate can be produced. The cross-linked lithium polyacrylatesheet may also be formed in situ, for example, by the replacement of thesodium ion (of a sodium polyacrylate) with a lithium ion within alithium electrolyte or battery. This cross-linked lithium polyacrylateseparator may be especially compatible with the lithium ionelectrochemical cells, facilitating the transport of the lithium ionsbetween electrodes and, in so doing, reducing ionic resistivity.

The following examples are given for illustrative purposes only and arenot meant to be a limitation on the invention described herein or on theclaims appended hereto. All parts and percentages given in thedescription, examples and claims appended hereto are by volume unlessotherwise stipulated. Further, all ranges of numbers provided hereinabove shall be deemed to specifically disclose all subset ranges ofnumbers within each given range.

In the case of electrochemical cell separator, the guiding principle fordetermination of optimal composition is highest conductivity in alkalineelectrolyte while demonstrating desirable physical and mechanicalcharacteristics. All samples below were processed similarly, with thematerial mixture processed in prior described sequence via a co-rotatingtwin screw extruder. The extruder was set at a temperature of 100° C. atthe feed zone, 200° C. at the melt zone, the extruder was vented priorto the pumping section, the extruder pumping section and die were set at180° C. The melt extrudate was cast onto a cast roller set at 40° C.,with the total extrusion rate of 4 kg/hr, the cast roller having atakeoff speed of 4 ft/min.

Materials: Materials Used in the Formation of the Sheet Product Include:

Polyethylene—LLDPE GA601 from Lyondell Basell or HDPE 2908 from NovaChemicalsPolypropylene—F006EC2 from BraskemPolyamide—Nylon Grilamid L25 from EMS Chemie AGSuperabsorbent Polyacrylate Materials from Arkent or AquasorbSilica—Sipernat 50s from EvonicMineral spirits—Kaydol from SonnebornEVA NA362006 from Lyondell BassellPolyox—WSE308 Polyethylene oxide from Dow ChemicalPEG 600 from Dow ChemicalSpan 80 and Tween 60 surfactants from Croda

Equipment:

Balance—OHaus I-10 2.5 kg balanceBlender—Ross planetary blender, Hobart, model 3943Fluid pump—Neptune, model 515AN3Screw feeder—K-Ton Corp., model K2MVS60Extruder—Coperion twin screw, model ZSK30Cast film take-up—Davis Standard CompanyBi-orientation tenter frame—Marshall and Williams

Compositions with a cross-linked polyacrylate superabsorber wereexamined for single layer membranes with a principal backbone ofpolyolefin or polyamide. Multilayer sheets were made with polyacrylatesuperabsorber contained in the inner layer, and outer layers formed frommicroporous polyamide, polyolefin, or combinations thereof.

Example 1

Each of samples A1, A2, A3, B1, B2 and B3 represents a membranecomprising polyamide and superabsorber. The composition and performanceof each sample are shown below in Table 1.

TABLE 1 Sample A1 A2 A3 B1 B2 B3 Polyamide 6 or 612 66% 66% 66% 59% 59%59% Superabsorbent 13% 13% 13% 17% 17% 17% Silica  3%  3%  3%  2%  2% 2% Surfactant 16% 16% 16% 11% 11% 11% EVA NA362005  0%  0%  0%  8%  8% 8% Polyox  2%  2%  2%  3%  3%  3% Total 100%  100%  100%  100%  100% 100%  Stretch treatment, %  0% 300%  300%   0% 300%  300%  TD TD TD TDOriginal thickness, mm 0.180 0.132 0.102 0.390 0.265 0.200 Original wt,mg/cm² 7.9 4.6 4.0 26.8 12.8 9.9 Average weight gain, 24 h RT, % 46% 42%−9% 37% 81% 37% Average thickness gain, 24 h RT, % −11%   7% 115%  −1%14% 97% Weight gain after 70° C., % 36% 32% −3% 35% 62% 27% Thicknessgain after 70° C., % −8%  8% 240%  −2% 21% 172%  Resistance, R, in 30%KOH, Ω cm² 6.8 8.3 6.8 10.4 3.2 2.8 Resistivity, ρ, Ω cm 424 614 615 276109 119

Thickness was measured using a Mitutoyo 1D-C112EXB Thickness Gauge.Thickness gain percent of the separator was calculated by measuring thethickness before and after soaking in an electrolyte liquid. Similarly,the weight gain percent of the separator sample was calculated bymeasuring the weight of a sample before and after soaking (for aspecific time) in an electrolyte liquid.

The resistivity of the separator was measured by placing a separator ina pair of electrodes immersed in 30% KOH electrolyte. The electrodeswere connected to an HP 4338B Milliohm Meter. When the resistance wasmeasured by the HP meter, with and without a separator, this differencewas the resistance, recorded in ohm-cm-sq. By dividing the resistance bythe thickness of the separator, this normalized the reading, or recordedin ohm-cm as the resistivity of the separator.

Example 2

Each of samples C1, C2, D1, D2, E1 and E2 represents a membranecomprising polyolefin and superabsorber. The composition and performanceof each sample are shown below in Table 2.

TABLE 2 Sample C1 C2 D1 D2 E1 E2 LLDPE, GA601 66% 66% 68% 68% 51% 51%Superabsorbent 17% 17% 21% 21% 17% 17% Silica  2%  2%  1%  1%  4%  4%Surfactant  6%  6%  2%  2% 14% 14% Mineral spirit  6%  6%  3%  3%  4% 4% EVA NA362005  0%  0%  0%  0%  8%  8% Polyox  3%  3%  6%  6%  3%  3%Total 100%  100%  100%  100%  100%  100%  Stretch treatment, %  0% 300%  0% 15%  0%  0% TD TD Original thickness, mm 0.225 0.150 0.345 0.3400.320 0.341 Original wt, mg/cm² 12.8 5.6 20.0 21.4 17.6 19.7 Averageweight gain, 24 h RT, %  6% 47% 22% 31% 34% −6% Average thickness gain,24 h RT, % −12%  10% −10%  −5% −9% 21% Weight gain after 70° C., %  0%27%  7%  6% 13% −18%  Thickness gain after 70° C., % −11%  33% −10%  −7%−10%  49% Resistance, R, in 30% KOH, Ω cm² 19.4 7.2 9.1 8.2 2.0 1.5Resistivity, ρ, Ω cm 984 418 294 253 73 49

Example 3

Each of samples F1 and F2 had a co-extruded trilayer (BAB) structure.The inner layer (A) comprised polyamide as a principal component, withadded cross-linked polyacrylate superabsorber, silica, surfactant, andpolyox. Each of the outer layers (B) lacked superabsorbent and comprisedpolyamide, silica, surfactant and polyox. The co-extruded layers wereadhered together at the extrusion die opening and were stretched andtreated as a single membrane subsequently. This configuration ofstructural layers has the advantage of different properties on thesurface or in the interior of the membrane. The inner layer of thestructure absorbs electrolyte readily, allowing facile diffusion throughthe microporous outer layers. The microporous outer layers areprotective layers that keep superabsorbent in and foreign materials out.The composition and performance of each sample are shown below in Table3.

TABLE 3 Sample F1 F2 Layers in ABA structure A B A B Polyamide 6 or 61280% 61% 80% 61% Superabsorbent  0% 17%  0% 17% Silica  2%  3%  2%  3%Surfactant  8% 16%  8% 16% Polyox 11%  2% 11%  2% Total 100% 100% 100%100% Stretch treatment, % None 300% TD Original thickness, mm 0.27 0.20Original wt, mg/cm² 18.6 9.5 Average weight gain, 24 h RT 29% 21%Average thickness gain, 24 h RT 25% 85% Weight gain after 70 C, %  8%23% Thickness gain after 70 C, % 13% 18% Resistance, R, in 30% KOH, Ωcm² 16.5 13.9 Resistivity, ρ, Ω cm 594 591

Example 4

Each of samples G, H, I1, I2, J1 and J2 represents a membrane lacking asuperabsorber. The composition and performance of each sample are shownbelow in Table 4. One may note the higher resistivity exhibited by thesesamples lacking the SAP.

TABLE 4 Sample ID G H I1 I2 J1 J2 Formulation v % Polyamide 6 or 612 73%64% 56% 56% 72% 72% LLDPE, GA601  0%  0%  4%  4%  0%  0% Superabsorbent 0%  0%  0%  0%  0%  0% Silica  7%  9%  0%  0%  3%  3% Surfactant  0% 0%  0%  0% 14% 14% Polyox 20% 27% 40% 40% 11% 11% Total 100%  100% 100%  100%  100%  100%  Stretch treatment, %  0%  0%  0% 215%   0% 300% TD TD Original thickness, mm 0.122 0.098 0.070 0.040 0.190 0.063Original wt, mg/cm² 14.8 12.1 4.9 2.6 11.3 4.5 Average weight gain, 24 hRT, % NA NA −5% 17% 12% 49% Average thickness gain, 24 h RT, % NA NA  5% 0% −8% 32% Weight gain after 70° C., % NA NA −6% 19%  9% 31% Thicknessgain after 70° C., % NA NA 34%  7% −8% 51% Resistance, R, in 30% KOH, Ωcm² 78.9 29.5 15.1 25.6 25.1 8.0 Resistivity, r, Ω cm 6470 3010 23976027 1392 951

Example 5

Each of samples L1, L2, M1 and M2 represents a membrane comprisingpolyamide and superabsorber. The membranes showed ability to gain inweight and thickness when in contact with various liquids. A gain withliquid smoke flavor extract is unexpected. The composition andperformance of each sample are shown below in Table 5.

TABLE 5 Material ID L1 L2 M1 M2 TDO stretch % none 50% none 25%Formulation: SAP v % 23% 23% 17% 17% PA v % 70% 70% 59% 59% EVA v %  0% 0%  8%  8% Polyox v %  0%  0%  3%  3% Silica v %  0%  0%  2%  2%Surfactant v %  7%  7% 11% 11% Total 100%  100%  100%  100%  AbsorbencyWeight gain % (30% KOH, pH 15)  94% 116%  31% 65% Weight gain % (waterpH 6) 227%  184%  99% 119%  Weight gain % (acetic acid pH 4) 84% 76% 32%52% Weight gain % (Liquid smoke pH 5) 72% 68% 53% 82% Weight gain %(EC/DEC)  4% 70%  3% 54% Weight gain % (mineral spirits)  3%  8%  3% 12%Original thickness (mm) 0.37 0.33 0.48 0.35 Thickness gain % (30% KOH)22% −3% −3% 19% Thickness gain % (water pH 6) 59% 60% 49% 56% Thicknessgain % (acetic acid pH 4) 32% 32% 25% 22% Thickness gain % (L smoke pH5) 23% 21% 23% 18% Thickness gain % (mineral spirits)  2%  5%  2%  3%Resistance in 30% KOH Ω-cm{circumflex over ( )}2 3.7 3.4 8.4 3.6Resistivity Ω-cm 66 112 228 146 Dry Porosity (%)  4%  9%  4% 13% WetWater Porosity (%) 69% 65% 50% 54% Wet 30% KOH Porosity (%) 42% 47% 19%34% Dry Mean Pore Size (micron) NA 2.7 NA NA Wet (water) Mean Pore Size(micron) NA <0.07 NA NA SAP migrated to separator surface in water (aswt % of sample) 1.8% NA 12.7% NA in 30% KOH (as wt % of sample) 17.6% NA12.5% NA

The superabsorbent migration of the separator was measured by soakingthe separator sample in a specific electrolyte for a prescribed time andtemperature. The SAP migrated from within the separator onto the surfaceof the separator and solvated in the electrolyte. The surface SAP wasscraped off the separator with a spatula after soaking. The separatorsample was then dried at 120° C. The weight loss of the dry sample,before soaking and after removal of the surface SAP, was accounted foras the amount of the SAP that migrated from within the separator ontothe surface of the separator.

It may be noted that sample L2 had different absorption weight gainswith different electrolytes and solvents, ranging from 8% for mineralspirits to 184% for water. The 8% weight gain for mineral spiritsreflects the amount of porosity of the existing dry separator sample,without electrolyte and the associated SAP swelling. By contrast, the184% weight gain reflects the absorption of electrolyte by the SAP,increasing both the weight and the thickness of the separator. Thisseparator is also capable of absorbing liquid smoke extract to a weightgain of 68% and EC/DEC lithium ion electrolyte to a weight gain of 70%.It may also be noted that the separator thickness for sample L2increased by 60%.

Sample L2 also exhibited a pore size of 2.7 microns before exposure towater whereas, after exposure to water, the pore size decreased to below0.07 micron. This is believed to have occurred because, after absorptionof water by the superabsorbent polymer, the superabsorbent polymerswelled and filled the separator capillaries with the swelled SAP,creating nanopores through the SAP in the process. The separatorporosity was calculated to be 65% with water and demonstrated aresistivity of 112 ohm-cm with 30% KOH electrolyte. The un-stretchedsample of L1 showed that, when soaked with water, 1.8% of the SAPmigrated from within the separator onto the surface of the separatorwhereas, when the same sample was immersed in 30% KOH electrolyte, 17.6%of the SAP within the separator migrated to the surface of theseparator. The L2 sample was processed with 7% surfactant having asolubility parameter above 11.

Example 6

Sample X represents a membrane comprising polyethylene and superabsorberwhereas each of samples Y and Z represents a membrane comprisingpolyamide and superabsorber. The composition and performance of eachsample are shown below in Table 6.

TABLE 6 Sample X Y Z Polyethylene, Nova 78 0 0 2908 (wt %) Polyamide,EMS 0 78 60 CF6S (wt %) SAP, AquaSorb 22 22 40 AS50F (wt %) Total (wt %)100 100 100 SAP Migration (30% KOH soaked for 2 days) Separator Initial7.144 5.775 7.831 Weight (g) Re-dried Weight (g) 7.025 5.542 6.474 SAPmigrated (from 1.7% 4.0% 17.3% separator to electrolyte) ResistanceMeasurement Instrument Baseline 5 5.1 5.1 (ohm-cm²) Average Sample 150.085.5 12.9 Measurement (ohm-cm²) Thickness (mm) 0.42 0.58 0.75 SeparatorResistivity 3571 1474 172 (ohm-cm)

Each of the above samples was observed under microscope after beingsoaked in KOH electrolyte and then re-dried. Sample X showed a majorityof the SAP as clear particles that were still embedded within theseparator. Sample Y showed a majority of the SAP particles became opaqueor that the SAP migrated, creating porosity from electrolyte swell.Sample Z showed significant opacity of the separator, with a significantmigration of SAP out of the pores, leaving micro craters.

The above data indicate that sample X (with PE and SAP) exhibited a KOHresistivity of 3571 ohm-cm, with 1.7 percent of the SAP being lost intothe electrolyte. By contrast, samples Y and Z (each with polyamide andSAP) exhibited a much lower resistivity. In particular, sample Z, with40% SAP, exhibited a significantly reduced resistivity and a much higherSAP migration onto the separator surface (17%).

The embodiments of the present invention described above are intended tobe merely exemplary and those skilled in the art shall be able to makenumerous variations and modifications to it without departing from thespirit of the present invention. All such variations and modificationsare intended to be within the scope of the present invention as definedin the appended claims.

What is claimed is:
 1. A microporous food packaging sheet productsuitable for contacting a food item, the microporous food packagingsheet product made by a method comprising melt-extruding an extrusionmixture to produce a sheet material in film form and then cooling thesheet material, the extrusion mixture comprising a thermoplasticpolymer, a superabsorbent polymer, and a compatibilizing agent, whereinthe compatibilizing agent comprises a food additive, the compatibilizingagent promoting mixing of the thermoplastic polymer and thesuperabsorbent polymer and forming, by phase-separation, micropores inthe sheet material, wherein the compatibilizing agent is retained insitu in the micropores of the sheet material, wherein the microporousfood packaging sheet product is a single layer, wherein the microporousfood packaging sheet product has capillary porosity, and wherein themicroporous food packaging sheet product, even without being stretchedand/or even without having the compatibilizing agent removed, comprisesan open-celled matrix of the thermoplastic polymer in which thesuperabsorbent polymer is dispersed.
 2. The microporous food packagingsheet product as claimed in claim 1 wherein the food additive comprisesliquid smoke extract.
 3. The microporous food packaging sheet product asclaimed in claim 1 wherein the compatibilizing agent further comprises asurfactant.
 4. The microporous food packaging sheet product as claimedin claim 1 wherein the compatibilizing agent further comprises an oil.5. The microporous food packaging sheet product as claimed in claim 1wherein the extrusion mixture further comprises a colorant.
 6. Themicroporous food packaging sheet product as claimed in claim 1 whereinthe thermoplastic polymer comprises one or more thermoplastic polymersselected from the group consisting of polyolefins, polyamides,polyethylene terephthalate, polyacrylics, and polyvinyl acetate.
 7. Themicroporous food packaging sheet product as claimed in claim 6 whereinthe thermoplastic polymer comprises one or more thermoplastic polymersselected from the group consisting of polyolefins and polyamides.
 8. Themicroporous food packaging sheet product as claimed in claim 7 whereinthe thermoplastic polymer is a polyolefin.
 9. The microporous foodpackaging sheet product as claimed in claim 8 wherein the thermoplasticpolymer is a polyethylene.
 10. The microporous food packaging sheetproduct as claimed in claim 7 wherein the thermoplastic polymer is apolyamide.
 11. The microporous food packaging sheet product as claimedin claim 1 wherein the superabsorbent polymer comprises one or moresuperabsorbent polymers selected from the group consisting ofcross-linked polyacrylates, methacrylates, polyacrylamides,carboxymethyl celluloses, polyvinyl alcohol copolymers, polyethyleneoxides, starch-grafted copolyacrylates or polyacrylamides, and ethylenemaleic anhydride copolymers.
 12. The microporous food packaging sheetproduct as claimed in claim 11 wherein the superabsorbent polymercomprises a cross-linked polyacrylate.
 13. The microporous foodpackaging sheet product as claimed in claim 12 wherein the cross-linkedpolyacrylate is a cross-linked lithium polyacrylate.
 14. The microporousfood packaging sheet product as claimed in claim 1 wherein thesuperabsorbent polymer is in particle form and has a particle sizesmaller than about 30 microns.
 15. The microporous food packaging sheetproduct as claimed in claim 14 wherein the superabsorbent polymer has aparticle size of between 1 to 10 microns.
 16. The microporous foodpackaging sheet product as claimed in claim 1 wherein the thermoplasticpolymer constitutes about 15-80% by volume of the extrusion mixture,wherein the superabsorbent polymer constitutes about 1-80% by volume ofthe extrusion mixture, and wherein the compatibilizing agent constitutesabout 1-80% by volume of the extrusion mixture.
 17. The microporous foodpackaging sheet product as claimed in claim 1 wherein the method furthercomprises stretching the cooled sheet material in at least onedirection.
 18. The microporous food packaging sheet product as claimedin claim 1 wherein the microporous sheet product has an average poresize below 5 microns.
 19. A method of packaging a food item, the methodcomprising: (a) providing a microporous food packaging sheet product,the microporous food packaging sheet product made by a method comprisingmelt-extruding an extrusion mixture to produce a sheet material in filmform and then cooling the sheet material, the extrusion mixturecomprising a thermoplastic polymer, a superabsorbent polymer, and acompatibilizing agent, wherein the compatibilizing agent comprises afood additive, the compatibilizing agent promoting mixing of thethermoplastic polymer and the superabsorbent polymer and forming, byphase-separation, micropores in the sheet material, wherein thecompatibilizing agent is retained in situ in the micropores of the sheetmaterial, wherein the microporous food packaging sheet product is asingle layer, wherein the microporous food packaging sheet product hascapillary porosity, and wherein the microporous food packaging sheetproduct, even without being stretched and/or even without having thecompatibilizing agent removed, comprises an open-celled matrix of thethermoplastic polymer in which the superabsorbent polymer is dispersed;and (b) contacting a food item with the microporous food packaging sheetproduct.
 20. The method as claimed in claim 19 wherein at least some ofthe food additive is desorbed from the microporous food packaging sheetproduct onto the food product.
 21. The method as claimed in claim 19wherein the food product comprises a meat.
 22. The method as claimed inclaim 19 wherein the food additive comprises liquid smoke extract. 23.The method as claimed in claim 19 wherein the compatibilizing agentfurther comprises a surfactant.
 24. The method as claimed in claim 19wherein the compatibilizing agent further comprises an oil.
 25. Themethod as claimed in claim 19 wherein the extrusion mixture furthercomprises a colorant.
 26. The method as claimed in claim 19 wherein thethermoplastic polymer comprises one or more thermoplastic polymersselected from the group consisting of polyolefins, polyamides,polyethylene terephthalate, polyacrylics, and polyvinyl acetate.
 27. Themethod as claimed in claim 26 wherein the thermoplastic polymercomprises one or more thermoplastic polymers selected from the groupconsisting of polyolefins and polyamides.
 28. The method as claimed inclaim 19 wherein the superabsorbent polymer comprises one or moresuperabsorbent polymers selected from the group consisting ofcross-linked polyacrylates, methacrylates, polyacrylamides,carboxymethyl celluloses, polyvinyl alcohol copolymers, polyethyleneoxides, starch-grafted copolyacrylates or polyacrylamides, and ethylenemaleic anhydride copolymers.
 29. The method as claimed in claim 28wherein the superabsorbent polymer is a cross-linked lithiumpolyacrylate.
 30. The method as claimed in claim 19 wherein thethermoplastic polymer constitutes about 15-80% by volume of theextrusion mixture, wherein the superabsorbent polymer constitutes about1-80% by volume of the extrusion mixture, and wherein thecompatibilizing agent constitutes about 1-80% by volume of the extrusionmixture.
 31. The method as claimed in claim 19 further comprisesstretching the cooled sheet material in at least one direction.
 32. Apackaged food item made by the method of claim
 19. 33. The packaged fooditem as claimed in claim 32 wherein the food item comprises a meat. 34.The packaged food item as claimed in claim 33 wherein the food additivecomprises liquid smoke extract and wherein at least some of the foodadditive is desorbed from the microporous food packaging sheet productonto the food product.